I had already published a novel and it was an unexpected success. I thought my ___________ .

The speaker is saying that his novel was a success, but that is not what he/she expected. Option (d) is correct, because making a fortune means achieving a lot of success, prosperity or making lots of money. So, it refers to achieving success, which is what the context is. Option (a) is wrong, because days were up is an idiom which refers to ending or coming to an end, this ending could be of anything, like success, happiness etc. But the author, in fact, got success, so this option is logically incorrect. Option (b) is wrong because chances are not talked about after the result and similarly, option (c) can be ruled out too.